In recent years, an RFID system is commonly used in which data are transmitted between a transponder which is provided with an IC chip and a reader/writer (or a reader). In such an RFID system, data are transmitted by using antennae which are provided to the transponder and the reader/writer. Therefore, it is possible to perform data communication even if the transponder is disposed from the reader/writer by several centimeters to several tens centimeters. Also, there is an advantage to a dirty condition or a static electricity; thus, such an RFID system is commonly used for various usages such as a production control in a manufacturing facility, a material handling control, and an admission control.
Conventionally, a hollow coil has been used for an antenna which is used for a reader/writer and a transponder for such an RFID system because a hollow coil is inexpensive and has a superior performance. For such an antenna for such a hollow coil, a lead wire which is coated by an insulating layer is wound around a base plate in a spiral manner or a member which is formed by etching a metal layer such as an aluminum layer or a copper layer which are layered on a base plate are commonly known (Japanese Unexamined Patent Application, First Publication No. Hei 321190).
However, a magnetic flux is generated in a direction which penetrates through the base plate in the above explained hollow coil antenna; therefore, if the antenna coil contacts a metal object, the magnetic flux penetrates through the base plate so as to reach to the metal object. By doing this, an eddy current is generated in the metal member; thus, there are problems in that a resonation frequency in a resonating circuit which is formed by an antenna coil and a condenser varies, or a loss increases.
In particular, a reader/writer is different from the transponder in that the reader/writer needs many electronic circuits such as a circuit for driving the reader/writer, a control circuit, and a power supply. These electronic circuits and a supporting member for supporting such circuits contain a lot of metal member; therefore, in the reader/writer antenna, there occur a variance for the resonating frequency and a loss may increase due to receiving an influence of these metal members. Therefore, a structure is used for the conventional reader/writer in which the antenna and the electronic circuit are contained in different cases such that a cable connects therebetween.
However, in such a structure, it is necessary to separate the casing for the electronic circuits and the casing for the antenna every time a device is carried and connect them every time they are used. Such a condition is not easy from a handling point of view. Also, it is preferable to form the small, rigid casing by a metal member such as an iron, aluminum, and a magnesium for a carrying purpose. However, if the hollow coil antenna is disposed so as to contact the casing, the variance for the resonating frequency and the loss may increase greatly. Also, if the case is made from a plastic member and there is a disposition layer such as an aluminum, or a metal member such as an aluminum flake contained in a filling member or a painting layer, then a loss in the coil may increase greatly.